It appears that Luca and his mates, with who you collected the diamonds in Taking in the Trash, never delivered them to Ray. That is why Ray wants you to go to Luca and get the diamonds back. So go to Castle Gardens City, where Luca hangs out often, and step into the marker. Niko will ask Luca about the diamonds after which Luca will flee. Niko will walk towards a Banshee which was parked behind Luca’s car and enter it. Chase Luca and his friends. You have to keep Luca alive. Follow them all the way to Middle Park, where they will get an accident and go on on foot. Kill Luca’s friends and chase Luca. Luca will hide in one of the toilets. Shoot at the doors to open them until you find Luca. Once you have found Luca, select your pistol and execute him. Take the diamonds and go outside. Niko now will call Ray to tell him everybody is dead and that Ray can get his diamonds back on the bridge in Middle Park. So go to the bridge and deliver the diamonds to Ray.

Trivia

The traffic is always the same. You will see a Laundromat, which is a special edition of the Burrito. It will only appear twice in this game; the other one is the one you have to chase during the mission Hung Out to Dry.